1. Field of the Invention
This invention relates to a process for producing gel-coated, fiber-reinforced shaped articles wherein the total processing time is substantially decreased compared to conventional processes, and the physico-mechanical properties of the gel-coated, shaped articles produced therefrom are maintained.
2. Discussion of the Background
Gel-coated, fiber-reinforced shaped articles are conventionally used to produce bathroom fixtures such as bath tubs, sinks and shower stalls; recreational applications such as boat hulls, canoes and kayaks, as well as panels, especially for roofing, walls and doors of industrial buildings or recreational vehicles.
Gel-coated fiber-reinforced shaped articles are usually prepared from thermosetting resins, in particular from unsaturated polyester resins prepared by polycondensing diols and polycarboxylic acids, either or both of which contain reactive double bonds. The so-obtained product is then mixed with a reactive monomer, e.g., styrene, in order to have cross-linking during polymerization.
When the resin is to be used, filler, e.g., glass fibers or inorganic powders are mixed with it. The amount and kind of filler affect the strength, flexibility, and cost of the product. Thixotropic agents may be used to control the viscosity and prevent the mix from draining from sloping surfaces. Pigments can be used to provide color while UV absorbers can be used to provide outdoor stability. A peroxide catalyst and a combination of a cobalt salt and, possibly, a tertiary amine are then added as accelerators for the crosslinking reaction between the unsaturated polyester and the reactive monomer.
In fabricating jacuzzis, shower stalls, bath tubs, etc., a layer of polyester resin system as above described, containing no fiber reinforcement, is applied first to the surface of a mold (gel coat). It forms a smooth, strong, impervious, durable chemical-, weather-, and wear-resistant surface. An additional, thicker layer of the resin, which is usually reinforced with glass fiber, is then applied by hand lay-up or by spray gun. The article is then cured at room or higher temperature.
More particularly, the preparation process consists of:
(a) spraying onto the surface of a mold, previously treated with releasing agents, a first layer (gel-coat) of an unsaturated polyester resin system (polyester resin plus catalyst plus additives) which forms the external surface of the molded article; PA1 (b) partially curing the gel-coat (this step usually requires about 40 minutes); PA1 (c) spraying onto the partially cured gel-coat layer a second and thicker layer of an unsaturated polyester resin system mixed with chopped glass fibers; and PA1 (d) further curing the polyester resin, both of the first and the second layer, in an oven at about 200.degree. F. (this step usually requires an additional 80-90 minutes). The total cycle time for such a preparation is about two hours. PA1 a) applying onto the surface of a mold, previously treated with releasing agents, a gel-coat layer of an unsaturated polyester resin system (this layer will represent the external surface of the molded articles); PA1 b) applying onto the gel-coat layer, after 5-7 minutes, a "backer layer" consisting essentially of: PA1 c) applying onto the backer layer, after 5-8 minutes, a layer consisting of a polyurethane system optionally mixed with reinforcement materials; and PA1 d) curing the so-obtained composite; wherein the total processing time for steps (a)-(d) is about 12 to 25 minutes, preferably 14-20 minutes.
It is desirable to decrease the total cycle time for such processes. Unfortunately, the curing time in either of steps (b) or (d) cannot be reduced. When the process time of step (b) is reduced, insufficient curing results and the chopped glass fibers tend to penetrate the gel-coat layer leading to a worsening of the aesthetic properties of the external surface of the article. When the process time of step (d) is reduced, for example by replacing the polyester resin system of (c) with a faster curing system such as a polyurethane system, the polyester gel-coat layer and the underlying polyurethane layer do not adequately adhere to each other, leading to inadequate physico-mechanical properties of the obtained articles.